In a continuing effort to simplify the use of electric arcs with metal wire for flame spraying purposes, the applicant has discovered that the optimum situation results when the arc intersects the wire (usually as the anode) within a region characterized by a quiescent atmosphere. Any significant motion of the ambient gases surrounding the wire can violently effect arc action, even extinguishing it. To date, the only practical, arc-wire system utilizes two separate wires. The wires are continuously fed toward each other and an arc is formed between their ends by subjecting a potential difference across the wires. A blast of compressed air directed at the arc is used to accelerate the molten material to the required velocity to form coatings. The air blast disrupts the arc, it is believed that the wires momentarily touch to cause a short-circuit. The wire melts back causing a short arc of very short life. The wires again approach each other and the process repeats many times per second. The arcing action is very unsteady with the result that a very wide range of molten particles are produced. This, in itself, is not bad. But, the presence of oversized particles diminishes coating quality.
The two-wire system has a second disadvantage in that one of the wires must be the cathode. Cathode temperatures can be much higher than that at the anode. In some cases an appreciable overheating of the metal causes a change in metallurgy as well as vaporizing a small amount of it.
With these drawbacks of the two-wire system in mind, attempts were made to produce an arc-wire system capable of achieving top quality coating while using only a single wire serving as the anode. An attempt was made to heat the wire used as an "open" welding arc such as that produced by a tungsten-inert gas ("T.I.G."). The arc was directed against the moving wire from a fixed cathode and the anode spot would affix itself to a particular heated point on the wire drawing the arc, to a length such that required voltage became greater than that provided by the power supply. As such, the arc went out even in a quiescent atmosphere. It soon became apparent that a much "stiffer" arc column was required; one which would keep the anode spot and arc column in a fixed spatial geometry.
It became apparent that to solve the wandering arc problem it was necessary to turn to commercially available transferred plasma-arc cutting equipment. The applicant found that the plasma-arc equipment utilizes a small flow of gas (such as nitrogen) to position and direct the arc column through and beyond the torch nozzle and even though this gas is raised to extreme temperatures and possesses high velocity, it was determined that there is simply not enough of it to act as an effective accelerator for molten materials. Further, attempts to use plasma-arc as a melter, passing a high velocity flow of compressed air axially along the wire, failed. Again, the arc voltage jumped to unsustainable levels with termination of the arc. From these tests, it became apparent that the plasma-arc and the anode spot were subjected to the gaseous accelerating medium and the interference was fatal.
It is, therefore, a primary object of the present invention to provide an arc-wire flame spray system which separates the arc melting of the wire from the gas flow used to accelerate the molten particle to velocities adequate for forming superior flame spray coating, or to equivalent systems and to effect introduction of the flame spray particle into the accelerating medium without any interference by the gaseous accelerating medium to the flame spray source.
It is a further object of the invention to provide an arc-wire flame spray system in which, the accelerating medium is provided with an inherent quiescent zone at the point of contact with the flame spray stream emanating from the plasma source so as to envelope the flame spray without interference by the gaseous accelerating medium.